1. Technical Field
The present invention relates to a substrate structure and a manufacturing method.
2. Related Art
As an optical switch and an optical modulator made of a ferroelectric material, Mach-Zehnder modulators mainly made of LiNbO3 single crystal and having a modulation frequency exceeding 40 GHz are already available in the market. For example, Patent Document No. 1 and Patent Document No. 2 report an optical switch having a vertical device structure which applies an electric field in the direction vertical to a surface of a ferroelectric thin film made of such as lead lanthanum zirconate titanate (composite oxide of PbLaZrTiO system, hereinafter abbreviated as “PLZT”).    Patent Document No. 1: Japanese Patent Application Publication No. 2001-117059    Patent Document No. 2: Japanese Patent Application Publication No. 2006-58837
However, the electro-optic coefficient of a LiNbO3 single crystal is relatively small (e.g. 30 pm/V), and so the operating voltage can not be reduced unless the length of the device can be made no smaller than about 30 mm. Moreover, in a modulator made of LiNbO3 single crystal (hereinafter referred to as “LN modulator”), a waveguide is fabricated by depositing a Ti thin film on the LiNbO3 single crystal substrate, and then subjecting it to thermodiffusion. Such a Ti diffused waveguide has a weak optical confinement effect because of a small difference in refractive indexes between the core and the clad compared to that of the case of a ridge waveguide, and so undergoes a larger loss in response to sharp bending. This has made it difficult to manufacture a small-sized LN modulator or integrate it.
In addition, the PLZT thin film has a relative dielectric constant as large as several hundreds to several thousands, and in fact the SrTiO3 substrate used in Patent Document No. 1 and Patent Document No. 2 has a large relative dielectric constant such as 300. Consequently, an optical switch and an optical modulator made of a ferroelectric material, expected to be manufactured smaller, end up having a large capacitance, thereby making it difficult to realize the modulation bandwidth no less than 10 GHz common in optical communication.